Studies on cell differentiation have revealed that 5-BUdR, at concentrations having little effect on growth and division, can reversibly inhibit the synthesis of tissue-specific substances. Similar concentrations of 5-BUdR have also been reported to suppress malignancy in mouse melanoma cells. Consequently, this laboratory has begun to study the effects of 5-BUdR on virus-transformed mouse cells to see if new insight might be gained into the contributions of the virus and host genomes to the transfer state. Specifically, the properties being studied include: (1) The effect of 5-BUdR or the tumorigenicity of virus-transformed cells; (2) The extent of expression of the virus genome in transformed cells compared to that in similar cells treated with 5-BUdR; (3) Whether differences in the transcription of the cell genome occur between normal, transformed, and 5-BUdR-treated transformed cells with regard to both repeated and unique DNA sequences; (4) Whether there is a correlation between the effects of 5-BUdR and cAMP.